Optically active salts of lysine and butane-2-sulfonic acid



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orrrcALLY ACTIVE SALTS or LYSlNE AND BUTANE-Z-SULFONIC ACID Artirnr 0. Rogers, Lewiston, N.Y., assignor to E. L du Pont rle Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 28, 19 55 1 Serial No. 537,297

2 Claims. (Cl.260501) This invention relates to an improved methodfor producing amino acids in optically active form.

Optically active amino acids, such as the essential amino acids, are valuable as food supplements and for medicinal uses. Many different routes have been pro.- posed for synthesizingvarious of the essential amino acids in their optically active forms valuable for nutritional purposes. Such syntheses frequently involve the formation of an intermediate compound which is hydrolyzable to the DL-amino acid. The DL-acid is then usually resolved to separate the desired biologically active L-form.

The following steps, starting with the amino acidyielding intermediate, are typical of those most frequently used in the more economical of such synthetic routes:

resolving agent resolving agent (4) Resolving agent (6) Resolving agent (recycle to Step 3) (recycle to Step3) Free D-amino acid (7) l Mineral, acid eat '(racemization) 1 DL-arnino acid salt of mineral acid Free L amino acid (5) 1 +1101 L-amino acid hydrochloride Mi -s ash Free DL-amino acid (recycle to-Step 3) It is an object of theinvention to provide a new method for resolving amino acids. A further object isj to provide a method for converting a mixture of the D form and the L-form of an amino .acid to a single optically active form, e.g. for obtaining the L- form from a Die-mixture. Still another object is to provide new compounds of amino acids which are valuable intermediates'in the production of optically active amino acids. I t

A process such as .that indicated schematically jabove involves repeated transformations between different compounds of the amino acid in the stepwise advancement through the process. Such transformations, with the required intermediate separating steps, add materially to 0 the cost of the final optically active amino acid product.

It is a further object of the invention to provide a method I nited States Patent 2,934,561 Patented Apr. 26, 1960 ice of :the above general .type .which such transformations .are in large part eliminated. Astill further object is to providea method in.which a single reagent is employed ;in the hydrolysis, resolution and racemization steps e.g., .Steps 1, 3.and 7 in the above general method, permitting the elimination of .various intermediate steps and even .an optically activesulfonic acid having a sulfonic acid group on a carbon atom of anacyclic aliphatic nucleus of not more than 10 carbon atoms. This is done by reacting the optically active sulfonic acid with a mixture of ,the' Dr and Leforms of an amino acid and fractionally crystallizing the resulting mixture of the D- and L-amino acid. saltsof the sulfonic acid from a suitable solvent. In a preferred .embodiment of .the invention, an optically tactivessulfonic acid of thetype indicated is employed as the single reagent for carrying out the hydrolysis, resolution and racemization steps in a method of the general type indicated schematically above. In a still further embodiment of the invention the optically active sulfonic .acid isemployed as the reagent in all three of such steps, and the hydrolysis and racemization steps (Steps 1 and 7) .are combined intoone operation.

Should it be desiredto employ the optically active sul- .fonic.ac id only as the resolving agent, the source of the .mixtureof the D- and L-forms of the amino acid which is to be resolved can be obtained in any desired manner. If obtained by hydrolyzing an amino acid-yielding intermediate, .hydrolysis may beLeffected employing either an .acidic or an alkaline hydrolyzingagent. However, .if the .hydrolyzing agent is to be an acid, 'it will generally be advantageous to employ the optically active sulfonic acid .to effectboth hydrolysis and resolution.

:The sulfonic acids useful in practicing the invention :arethe optically active sulfonic acids having a sulfonic acid group on a carbon atomof an acyclic aliphatic nucleus ofnotmore than 10 carbon atoms. Preferred examples are butane-Z-sulfonic acid and 2-methyl-butane-1- sulfonic acid. Other usable compounds are pentane-Z- sulfonic acid, 3-methylbutane-2=sulfonic acid, Z-methylpentane-l-sulfonic acid and 3-methylpentane-1-sulfonic acid. Compounds such as l-phenylethane-l-sulfonic acid and 2-phenylpropane-l-sulfonic acid, which have aromatic substituent groups attached to the aliphatic nucleus containing the sulfonic acidgroup, are also usable but are less preferred. Other substituent groups may also be present provided they are inert towards the amino acid. Examples are the ether (e.g. methoxy, ethoxy or phenoxy), hydroxyl, hydroxyalkyl, hydroxyphenyl, olefinic and cycloaliphatic groups. In general, it is only neccleus with the attached sulfonic acid group will contain not more than 6 carbon atoms. Butane-Z-sulfonic acid and Z-methylbutane-l-sulfonic acid are particularly preferred for reasons of economy and availability and for their efficiency in use.

A compound of the above type can be employed simply as the resolving agent in the resolution step of a general method such as that outlined above for producing optically active amino acids. Preferably, the cornpound will be employed as the active'agent in each of the hydrolysis, the resolution and the racemizationsteps; In so doing, a suitable'intermediate which is hydrolyzable to the DL-form of the desired amino acid is obtained or synthesized 'by any available means. Examples of such intermediates which have been used in various amino acid syntheses are: 5-(4-aminobutyl) hydantoin (for lysine); poly-3,5-tetramethylene hydantoin (for lysine); a-amino-'y-methylmercaptobutyronitrile (for methionine); ethyl a-acetamido-fi-indolylpropionate (for tryptophan); and a-amino e-benzamidocaproic acid (for lysine). The selected intermediate is then hydrolyzed by heating an aqueous solution thereof with the chosen optically active sulfonic acid to yield a solution of the D- and L-amino acid salts of the sulfonic acid in equal amounts. By suitable adjustment of the solvent composition and temperature, the salt of the desired amino acid isomer is crystallized out and is separated in substantially pure form, while the salt of the undesired isomer remains in the mother liquor. This operation comprises the resolu-- tion step.

The precipitated salt is subjected to any suitable treatment, e.g., by treating an aqueous solution thereof with an ion-exchange material, to separate the desired optically active amino acid and recover the sulfonic acid for recycling. The free optically active amino acid so-obtained may be employed as such, or it may be converted by known methods to its hydrochloride or to other known salts.

The mother liquor from the above resolution step, or

the amino acid salt separated therefrom, is heated to convert the undesired isomer of the amino acid to a mixture of the two optical isomers, which mixture is then separately fractionally crystallized or, preferably is recycled to the above resolution step. By repeating the over-all process, the DL-amino acid formed in the original hydrolysis step is ultimately completely converted to the desired optically active isomer.

In a further embodiment of the invention, the intermediate mother liquor from the resolution step containing the sulfonic acid salt of the undesired amino acid I terials similarly form two salts:

isomer, is recycled to the original hydrolysis step instead of being separately heated to effect racemization. In such an operation, racemization of the recycled material and hydrolysis of fresh amino acid-yielding intermediate are combined into a single operation. The resulting solution is a solution of the D- and L-amino acid salts of the sulfonic acid which is resolved by fractional crystallization from a suitable solvent as indicated above.

The foregoing operations require the use of the sulfonic acid in an optically active form which crystallizes preferentially as a salt of the desired amino acid isomer. In most cases, the sulfonic acid is synthesized from optically inactive materials and is, therefore, obtained in itially in racemic form, i.e., as a mixture of equal parts of the D- and L-isomers. Such isomers can be separated employing any suitable resolving agent to obtain an optically active isomer for use in resolving an amino acid. Separation can be effected employing an optically active amino acid as the resolving agent. Thus, when using the desired optical isomer of an amino acid for this purpose, the conditions required will be essentially those re quired for the reverse resolution of the racemic amino acid with the optically active sulfonic acid. The reason for this similarity will be apparent from the following considerations:

When resolving the racemic amino acid using the L- sulfonic acid as resolving agent to isolate the L-arnino acid, the resolution mixture will comprise the L-sulfonic acid (L-SA), the L-amino acid (L-AA) and the D-amino acid (D-AA). These materials form two salts:

L-SA-L-AA L-SA-D-AA (II) Since it is desired to isolate L-AA in pure form, conditions (e.g. solvent composition and temperature) will be chosen under which I is less soluble than II and will, therefore, crystallize preferentially.

When resolving the racemic sulfonic acid using the L-amino acid as resolving agent, the resolution mixture L-SA L-AA D-SA- L-AA By comparison of the above two pairs of salts, it will be noted that I and I are identical. II and II are enantiomorphous and, therefore, are identical in most physical properties, including solubility. Thus, in each of the resolutions indicated, the salt L-SA-LAA will crystallize preferentially. By suitable subsequent treatment of this salt, both the L-sulfonic acid and the L-amino acid can be recovered in pure form.

The following examples illustrate application of the principles discussed above to levo-butane-Z-sulfonic acid and L-lysine.

Example 1 A mixture of Z-brOmobu-tane (1 mole, 137 g.), sodium sulfite (1.2 mole, 151 g.) and water (600 g.) was boiled under reflux with stirring for 16 hours, during which time the organic phase dissolved and the temperature of the boiling mixture rose to 98 C. The resulting solution, containing 0.98 mole of bromide ion, was diluted with water and passed through a bed of a cation exchange resin (a sulfonated copolymer of styrene and divinyl benzene) in acid form to remove sodium ions, The effluent was heated in a steam bath under reduced pressure until substantially free of water and hydrogen bromide. The syrupy liquid residue consisted chiefly of DL-butane- 2-sulfonic acid.

Example 2 Crude sulfonic acid obtained as in Example 1 and containing about 0.035 mole of sulfuric acid and about 0.4 mole of DL-butane-Z-sulfonic acid was diluted with water to ml., then mixed with a 39.0% solution of free L- lysine containing 0.47 mole of L-lysine. The resulting weakly acidic solution was dccolorized with charcoal, then evaporated to dryness in a steam bath under reduced pressure. The residue was extracted with a boiling mixture of methanol (500 ml.) and water (25 ml.). The extract was filtered to remove the L-lysine salt of sulfuric acid which is substantially insoluble in aqueous methanol of this composition. Evaporation of the filtrate gave a glassy residue of L-lysine DL-butane-2-sulfonate free of sulfate impurities. Its specific rotation in water at a concentration of 4 g. per 100 ml. of solution at 20 C. was +6.19. Analyses for carbon, hydrogen and nitrogen gave correct values for C H N O S, the empmcal formula for the lysine butane sulfonate.

Example 3 A sample (113.5 g.) of L-lysine DL-butane-Z-sulfonate prepared essentially as described in Example 2, was dlS- solved in a boiling mixture of ethanol (500 ml.) and water (40 ml.). The solution was cooled to 50 C. and held at that temperature with stirring for about 24 hours. The precipitate, comprising the L-lysine salt of levobutane-Z-sulfonic acid in the form of small needles, was filtered rapidly in a preheated Biichner funnel, leaving in solution the L-lysine salt of dextro-butane-Z-sulfonic acid. The filter cake (wet weight, 20 g.) was dissolved in water and the solution passed through abed of a cation exchange resin (a sulfonated copolymer of styrene and divinyl benzene) in acid form. The L-lysine portion of the salt was retained by the bed while the butanesulfonic acid portion remained in the effiuent. The effluent, after evaporation to a butanesulfonic acid concentration of 27.6 g./ 100 ml. of solution, had an observed rotation of 0.060 This corresponds to a specific rotation of -0.109 for the butane-Z-suifonic acid.

Example 4 The resolution of DL-lysine with levo-butane-Z-sulfonic acid obtained as described in Example 3, is eliected substantially as described in Example 3 except that DL- lysine is employed instead of L-lysine and levo-butane-2- sulfonic acid is employed instead of the DL-acid. The precipitate from the crystallization step is, as in Example 3, the L-lysine salt of levo-butane-Z-sulfonic acid, the D- lysine salt of levo-butane-Z-sulfonic acid remaining in the solvent. When a water solution of the precipitated salt is passed through the bed of cation exchange resin, the desired L-lysine component of the salt is retained on the resin and is subsequently eluted therefrom, by means of an ammonium hydroxide solution to give a solution which yields free L-lysine on evaporation. The levo-butane-Z- sulfonic acid component of the salt stays in the efliuent from the resin bed and can be readily recovered from such solution for reuse.

The conditions for effecting satisfactory resolution may vary considerably depending upon the particular amino acid being resolved and the sulfonic acid chosen as the resolving agent. These conditions should provide a sub stantial difierence in solubility between the diastereoiso mers being separated, e.g. by suitable choice of solvent, and must also allow suificient crystal growth for satisfactory filtration. Thus, in the resolution of DL-butane- 2-sulfonic acid with L-lysine, or in the corresponding resolution of DL-lysine with levo-butane-Z-sulfonic acid, the required difference in solubility is realized by the use of mixtures of ethanol and water as solvent. In this system, however, crystallization at low temperatures form an exceedingly fine gel-like precipitate which is diflicult to filter satisfactorily. This difficulty can be overcome by digestion at an elevated temperature as illustrated in Examples 3 and 4. The precise conditions necessary for best operation in specific instances cannot be accurately predicted but can be readily determined experimentally.

Use of an optically active sulfonic acid in the hydrolysis of an amino acid-yielding intermediate in accordance with the invention involves heating the intermediate with an aqueous solution of the acid in such an amount and of such a concentration as will maintain the hydrolysis mixture strongly acidic. The heating should of course be continued for a time sufficient to complete the hydrolysis. The time and temperature required will vary with the type of intermediate used. Thus, an alpha-amino nitrile intermediate may be hydrolyzed to corresponding amino acid under relatively mild reaction conditions while hydrolysis of a hydantoin intermediate may require much more vigorous treatment though employing the same bydrolyzing agent. In general, the conditions previously found suitable for efiecting such hydrolysis reactions with mineral acids such as hydrochloric or sulfuric acid will also be suitable when employing an optically active sulfonic acid in accordance with the present invention.

Racernization of an optically active amino acid in the presence of a sulfonic acid in accordance with the invention involves simply heating a solution of these substances, under pressure if necessary, until the optical activity of the amino acid is destroyed. The time and temperature required to effect racemization difiers with different amino acids but can readily be determined experimentally in any given case. When the time and temperature requirements for the hydrolysis and racemization reactions are about the same, it is feasible and usually advantageous to carry out both simultaneously in one reaction mixture. The present invention provides a new method for obtaining an optically active amino acid from an intermediate compound which hydrolyzes to the DL-form of the acid, and for obtaining an optically active form of a sulfonic acid of the above type from a DL-mixture thereof. The optically active amino acid salts of the present optically active sulfonic acids are new compounds which are valuable intermediates from which the desired optically active amino acid can be separated and the optically active sulfonic acid can be recovered for reuse. Use of the present sulfonic acids as the active agents in each of the hydrolysis, resolution and racemization steps of processes for obtaining amino acids from amino acidyielding'intermediates permits worthwhile economies in operations through elimination and/or combination of various steps normally required in processes of this type.

I claim:

1. An optically active salt of lysine and butane-Z-sulfonic acid.

2. The L-lysine salt of levo-butane-Z-sulfonic acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,556,907 Emmick June 12, 1951 2,603,651 Gaudry July 15, 1952 2,657,230 Rodgers Oct. 27, 1953 OTHER REFERENCES 

1. AN OPTICALLY ACTIVE SALT OF LYSINE AND BUTANE - 2- SULFONIC ACID. 